In knee replacement surgery, a surgeon places a first prosthesis on the distal end of the femur and a second prosthesis at the proximal end of the tibia. A surgeon should place the prostheses in an optimal way with respect to various parameters, for example, with respect to the gap between the prostheses throughout a range of motion. For example, if the gap between the tibia and femur is too small at any time during motion, the patient can experience painful binding. On the other hand, if the gap is too large, the knee joint can be too loose and become unstable.
Determining an optimal position and orientation of these prostheses, or implants, relative to the patient is difficult. Many factors must be taken into account—some quantitative, other qualitative. So called “surgical navigation” systems can help quantify certain plan parameters with high accuracy, but it is still up to the surgeon to balance these and other parameters, incrementally adjusting the plan often with a simple user interface. The user interface must be simple to use in the operating room, and therefore all the ramifications of any particular decision by the surgeon cannot be presentable easily without a complex interface. Medical personnel can use the surgical navigation system's computer to optimize the quantitative parameters, though it cannot know what the surgeon is thinking, and it cannot evaluate the qualitative parameters. In addition, two or more potential surgical plans can be equivalent when evaluated per the qualitative metrics, but it would require a surgeon to choose between these alternatives in light of the qualitative factors. The mental task of predicting the outcome of desired changes to the plan that are available via the user interface with respect to both the qualitative and quantitative metrics is a difficult one.
In light of this challenge, there exists a general need to provide a surgeon with easy to evaluate predicted results of various actions, and to allow the surgeon to choose one or more actions that can result in an improved placement of the implant on one or both of a femur and a tibia.